1. Field of the Invention
The present invention relates to an optical router which is disposed on an optical node basis, selects the optical path of an optical signal transmitting through such an optical transmission line as an optical fiber, and outputs the optical signal through the optical path, whereby an optical communications network is formed. More specifically, the present invention relates to an optical router capable of preventing optical signals from conflicting with each other and of efficiently allocating an optimum optical path.
2. Description of the Prior Art
An optical router is disposed on an optical node basis, selects the optical path of an optical signal transmitting through such an optical transmission line as an optical fiber, and outputs the optical signal through the optical path, whereby an optical communications network is formed. Literatures on the prior art related to optical routers include the following:    Japanese Laid-open Patent Application 1996-163610    Japanese Laid-open Patent Application 1996-204675    “Photonic Network Revolution—Technologies for realizing the world's most advanced information technology nations”: published by the Secretariat, Photonic Internet Forum, within The Support Center for Advanced Telecommunications Technology Research, Foundation (January 2002): 95-98
FIG. 1 is a block diagram illustrating an example of such conventional optical routers (optical burst switches, in particular) as mentioned above.
In FIG. 1, optical switch 1 having three input ports and three output ports is controlled by controller 2 in terms of optical path selection. Optical switch 1 and controller 2 compose optical router 50.
The main signals, which are the optical input signals (optical burst signals) indicated by MS01 in FIG. 1, are input to the three input ports of optical switch 1. The three optical output signals indicated by MS02 in FIG. 1 are output from the three output ports of optical switch 1 to a subsequent-stage optical router or the like.
An optical control signal carrying routing information indicated by CS01 in FIG. 1 is input to controller 2. The electrical output signal of controller 2 indicated by SS01 is coupled with the control terminal of optical switch 1. The optical output signal of controller 2 is output to a subsequent-stage optical router or the like, as the optical control signal indicated by CS02.
Now the behavior of an example of a conventional optical router illustrated in FIG. 1 is described by referring to FIG. 2, wherein FIG. 2 is a block diagram explaining the behavior of the example of the conventional optical router shown in FIG. 1. In FIG. 2, optical switch 1, controller 2 and optical router 50 are the same as those shown in FIG. 1. The details of interconnection are also the same as those of the example of the conventional optical router illustrated in FIG. 1.
First, an optical control signal indicated by CS11 in FIG. 2 is transferred prior to such a main signal (optical input signal) as indicated by SG11 in FIG. 2, and is input to controller 2.
Controller 2 then converts the optical control signal thus input, to an electrical control signal, as well as produces a control signal as indicated by SS11 in FIG. 2, according to the routing information that the electrical control signal carries, in order to select the optical path of optical switch 1.
Now assume, for example, that the abovementioned routing information contains the information that the main signal is input from the input port of optical switch 1 indicated by PT11 in FIG. 2, and is output from the output port of optical switch 1 indicated by PT12. Then, controller 2 controls optical switch 1 to select an optical path as indicated by SR11 in FIG. 2.
When such optical path selection as discussed above is completed, such a main signal as indicated by SG11 in FIG. 2 is input to the input port of optical switch 1 indicated by PT11. Consequently, the main signal is output from an output port indicated by PT12, as indicated by SG12 in the figure.
As a result, by previously selecting the optical path of optical switch 1 that composes optical router 50 using an optical control signal, then transmitting an optical input signal which is the main signal, it is possible to select and control the paths of optical signals.
However, the example of the conventional optical router illustrated in FIG. 1 has been problematic in that an optical control signal with a shifted transmission time must be separately transmitted in addition to the optical signal that is the main signal.
For this reason, the patent application 2002-284970 filed by the applicant of the application concerned describes an example of an optical router wherein an optical signal being transmitted is split into a header part and a data part and routing information, such as a destination address, is added to the header part according to the routing information, thereby permitting a selection to be made from given optical paths.
FIG. 3 is a block diagram illustrating an example of a prior art optical router described in the patent application 2002-284970.
Indicated in FIG. 3 are optical delay means 3 for delaying an optical input signal by a specific length of time by transmitting the signal through an optical loop or the like; optical-electrical conversion means 4, such as a photodiode or phototransistor; optical switch 5 provided with three input ports and three output ports; controller 6 for controlling optical path selection made by optical switch 5; and memory 7 wherein path control information, such as routing tables, is stored. In addition, optical delay means 3, optical-electrical conversion means 4, optical switch 5, controller 6 and memory 7 compose optical router 51.
In FIG. 3, the three optical input signals (optical burst signals) indicated by SG21 are input to the three input ends of optical delay means 3, as well as to the three input ends of optical-electrical conversion means 4.
Optical output signals from the three output ends of optical delay means 3 are input to the three input ports of optical switch 5, and the three optical output signals indicated by SG22 in FIG. 3 are output from the three output ports of optical switch 5.
The electrical output signal of optical-electrical conversion means 4 is coupled with controller 6, and the electrical output signal of controller 6 indicated by SS21 in FIG. 3 is coupled with the control terminal of optical switch 5. In addition, the electrical output signal of controller 6 is mutually coupled with memory 7.
Now the behavior of the example of the prior art optical router illustrated in FIG. 3 is described. Optical input signals, which are indicated by SG21 in FIG. 3 and contain routing information, such as destination addresses, added to the header parts thereof, are delayed by specific lengths of time at optical delay means 3.
The optical input signals indicated by SG21 in FIG. 3, which contain routing information, such as destination addresses, added to the header parts thereof, are converted to electrical signals at optical-electrical conversion means 4 and input to controller 6.
Controller 6 extracts the routing information from the electrical signal being input from optical-electrical conversion means 4, finds path control information stored in memory 7 according to the routing information, specifies a subsequent-stage optical router (output port) appropriate for the entered optical signals to transmit to the destination through the shortest path, and accordingly selects from the optical paths of optical switch 5.
For example, controller 6 controls optical switch 5 so that an optical path is selected in such a manner that an optical input signal is input to the input port of optical switch 5 indicated by PT21 in FIG. 3, and is output from the output port of optical switch 5 indicated by PT22.
If such an optical input signal as is properly delayed by optical delay means 3 after the completion of such optical path selection as described above is input to the input port of optical switch 5 indicated by PT21 in FIG. 3, the optical output signal will be output from the output port indicated by PT22.
This means that by adding routing information, such as a destination address, to the header part of an optical signal, it is possible to make optical path selections according to the routing information. In this case, it is not necessary to separately transmit an optical control signal as shown in the example of the prior art optical router illustrated in FIG. 1.
In such examples of the prior art optical router as illustrated in FIGS. 1 and 3, however, no consideration is given as to how to avoid a conflict between optical signals when a plurality of optical signals are input simultaneously. Consequently, a conflict between these optical input signals will occur if an attempt is made to output the simultaneously entered plural optical input signals onto the same output port. This example of the prior art optical router has been problematic since failures may occur such as a specific optical signal or signals not being transmitted at all (or being blocked).
Let us take FIG. 4 as an example, which is a block diagram explaining problems inherent in the example of the prior art optical router illustrated in FIG. 1, where optical switch 1, controller 2 and optical router 50 are identical with those shown in FIG. 1.
If two optical input signals indicated by SG31 and SG32 in FIG. 4 are simultaneously input to the input ports of optical switch 1 indicated by PT31 and PT32 and an attempt is made to output the two signals to the one and the same output port indicated by PT33, such a conflict as indicated by CL31 occurs, causing the abovementioned failure.
Let us take FIG. 5 as another example, which is a block diagram explaining problems inherent in the example of the prior art optical router illustrated in FIG. 3, where optical delay means 3, optical-electrical conversion means 4, optical switch 5, controller 6, memory 7 and optical router 51 are identical with those shown in FIG. 3.
If two optical input signals indicated by SG41 and SG42 in FIG. 5 are simultaneously input to the input ports of optical switch 5 indicated by PT41 and PT42 and an attempt is made to output the two signals to the one and the same output port indicated by PT43, such a conflict as indicated by CL41 occurs, causing the abovementioned failure.